You can add an image to a text field via a custom apperance (annot.SetAppearance()). Please, check the following forum postings:
https://groups.google.com/d/topic/pdfnet-sdk/0zKs-_nCjTs/discussion
https://groups.google.com/d/topic/pdfnet-sdk/HgnlxdtsdKE/discussion
https://groups.google.com/d/topic/pdfnet-sdk/DpS30pF5aXQ/discussion
https://groups.google.com/forum/?hl=en&fromgroups=#!searchin/pdfnet-sdk/SetAppearance
This post may also help you:
https://groups.google.com/d/topic/pdfnet-sdk/yKAdAs-RzKQ/discussion